The present invention relates to a thermal transfer sheet, and particularly to a thermal transfer sheet which is capable of forming an image of good quality on a transfer receiving paper even when the transfer receiving paper comprises a plain paper having a relatively rough texture (or relatively coarse grain).
Hitherto, in a case where an output from a computer, word processor, or facsimile, etc., is printed by a thermal transfer system, there has been used a thermal transfer sheet comprising a substrate film and a transferable ink layer disposed on one surface side thereof. Such a conventional thermal transfer sheet comprises a substrate film comprising a plastic film having a thickness of 3 to 20 .mu.m such as a polyethylene terephthalate film. The above mentioned thermal transfer sheet has been prepared by coating the substrate film with a transferable ink comprising a wax and a colorant such as a dye or a pigment mixed therein, to form a transferable ink layer on the substrate film (Japanese Laid Open Patent Application (JP A, KOKAI), 207682/1988, etc.).
However, the conventional thermal transfer sheet as described above has some disadvantages. One of these disadvantages is such that when an image is intended to be formed on a transfer receiving paper having a relatively rough texture, such as plain paper and regenerated paper, by use of such a thermal transfer sheet, the polyethylene terephthalate film has too high a strength and the printing pressure based on a thermal head used for the thermal transfer operation is not sufficiently transmitting to the concavities (or indentations) of the transfer receiving paper. As a result, there has been posed a problem such that the ink is not sufficiently transferred to the transfer receiving paper so that an image having a lot of so-called dropouts or image defects is provided, and therefore an iamge of high quality cannot be formed.
For the purpose of solving such a problem, these nave been made various proposals. For example, there has been proposed a method wherein the texture (or again) of a paper to be subjected to a recording operation is covered with a transferable ink by increasing the amount of the transfeable ink. However, when such a method is used, the thermal transfer sheet to be used for such a purpose is inevitably caused to have a large thickness so that the sensitivity thereof may be lowered. In order to solve such a problem, there has been proposed a method wherein the substrate film constituting the thermal transfer sheet is caused to have a smaller thickness so as to improve the sensitivity thereof and to obtain a proper sensitivity. In such a case, however, a polyethylene terephthalate film may generally have a poor thermal stability and may pose a problem such that a crease is liable to occur. Accordingly, it is necessary to improve such a film, and there have been made various proposals for the purpose of solving the above problems (e.g., Japanese Laid-Open Patent Application (JP-A, KOKAI) Nos. 104393/1985; 207682/1988; 39374/1988; 193889/1987; 191679/1988; etc.). However, when any of these methods is used, the resultant polyethylene terephthalate film has smaller thickness. Therefore it is necessary to treat such a film so that the resultant film may have a excellent sticking prevention property in consideration of excessive energy supplied thereto from a thermal head. As a result, the production cost in such case is increased. In addition, since the thickness of the transferable ink layer becomes larger, the improvement in the sharpness of the resultant printed image inevitably has a certain limit.
In addition, there has also been posed another problem such that since a polyester film having a high strength is used as the substrate film of a thermal transfer sheet, the used thermal transfer sheet is not sufficiently cut or shredded by means of a shredder so as to cause a trouble of the shredder, when the thermal transfer sheet is intended to be shredded by means of the shredder for the purpose of preventing the leakage of the secret based on the used thermal trasnfer sheet.